In order to thoroughly understand the purpose of the invention, it is appropriate to consider the main kinds of flying machine that correspond to airplanes and to rotorcraft. The term “rotorcraft” covers any aircraft in which lift is provided in full or in part by one or more propellers of substantially vertical axis, and of large diameter, known as rotors or indeed as rotary wings. In the rotorcraft category, various distinct types of rotorcraft are distinguished.
Firstly, there is the helicopter, in which at least one main rotor driven by a suitable engine serves to provide both lift and propulsion. Then, there is the autogyro, which is a rotorcraft in which the rotor is not powered, but provides lift by autorotation under the effect of the forward speed of the aircraft. Propulsion is provided by a turbine engine, or indeed by a propeller of axis that is substantially horizontal in forward flight, and driven by a conventional engine.
The gyrodyne is a rotorcraft intermediate between the helicopter and the autogyro in which the rotor provides lift only. The rotor is normally driven by an engine installation during stages of takeoff, hovering, vertical flight, and landing, like a helicopter. A gyrodyne also has an additional propulsion system that is essentially different from the rotor assembly. In forward flight, the rotor continues to provide lift, but solely in autorotation mode, i.e. without power being transmitted to said rotor.
Several other novel formulae have been studied to a greater or lesser extent, and some of them have given rise to practical embodiments.
In this respect, mention can be made of the compound rotorcraft that takes off and lands like a helicopter, and that performs cruising flight like an autogyro: its rotor moves by autorotation because of the forward speed of the aircraft and provides some of the lift, while the remainder of the lift is provided by an auxiliary wing. A tractor propeller with a substantially horizontal axis delivers the force needed for movement in translation.
Similarly, document U.S. Pat. No. 6,513,752 discloses an aircraft comprising:                a fuselage and a wing;        two variable pitch propellers;        a rotor with “end” masses;        a power source driving the two propellers and the rotor;        control means for adjusting the pitch of the propellers so that:                    in forward flight, the thrust from the propellers is exerted towards the front of the rotorcraft; and            in hovering flight, the antitorque function is provided by one propeller providing thrust towards the front and the other propeller towards the rear of the rotorcraft, with the rotor being driven by the power source; and                        the power source comprises an engine and a clutch that, by disconnecting the rotor from the engine, enables the rotor to turn faster than an outlet from said engine, because of the above-mentioned masses.        
It is also specified that the clutch enables the aircraft to fly in autogyro mode during forward flight. Consequently, the aircraft described in U.S. Pat. No. 6,513,752 is of the compound type. In addition, the power transmission gearbox located between the power source and the propellers enables said propellers to operate at a plurality of different speeds of rotation relative to the speed of an outlet of said power source.
A convertible rotorcraft constitutes another particular formula for a rotorcraft. This term covers all rotorcraft that change configuration in flight: takeoff and landing in a helicopter configuration, cruising flight in an airplane configuration, e.g. having two rotors that are tilted through about 90° so as to act as propellers. Another novel formula is known for convenience as a “hybrid helicopter”. A hybrid helicopter comprises a fuselage, a rotary wing provided with a main rotor for driving blades in rotation by means of at least one turbine engine.
In addition, a hybrid helicopter has a wing made up of two half-wings, with two propulsive propellers being placed on the half-wings on either side of the fuselage. Furthermore, a hybrid helicopter is fitted with an integrated drive system that comprises not only the turbine engine(s), the rotor of the rotary wing, and the two propellers, but also a mechanical system for interconnecting those elements.
With this configuration, the speeds of rotation of the outlet(s) from the turbine engine(s), of the propellers of the rotor, and of the mechanical interconnection system are mutually proportional, with the proportionality ratio being constant regardless of the flying configuration of the hybrid helicopter under normal conditions of operation of the integrated drive system. Consequently, and advantageously, the rotor is always driven in rotation by the turbine engine(s), and it always develops lift regardless of the configuration of the hybrid helicopter, both in forward flight and while hovering. A hybrid helicopter is thus neither an autogyro, nor a gyrodyne, nor a compound rotorcraft, but is a novel type of rotorcraft.
More precisely, the rotor serves to provide all of the lift for a hybrid helicopter during stages of takeoff, landing, and vertical flight, and to provide some lift in cruising flight, with the wing then contributing to some extent to support said hybrid helicopter. Thus, the rotor provides most of the lift of a hybrid helicopter in cruising flight possibility together with a small contribution to propulsion or traction forces, and always in a minimum drag configuration. The antitorque and yaw control functions are provided by exerting differential thrust with the propellers. For example, in vertical flight and assuming that the rotor turns clockwise, the propeller on the left of the fuselage exerts thrust towards the rear of the hybrid helicopter while the propeller on the right produces thrust towards the front. To pilot a hybrid helicopter, it is therefore appropriate to act on the pitches of the propellers of the hybrid helicopter.
Consequently, the flight controls of a hybrid helicopter act on servocontrols that are suitable for modifying the pitches of the propeller blades via control means that are controlled by the pilot or indeed by an autopilot. Like in airplanes, a first servocontrol is arranged in the left propeller hub to control its pitch, while a second servocontrol is arranged in the hub of the right propeller. Since the space available inside a hub is small, each servocontrol is itself controlled via an outlet rod of a hydraulic valve, the hydraulic valve delivering fluid to the servocontrol in order to modify the pitch of the blades of the corresponding propeller.
Consequently, the hydraulic valve is provided with a control rod that is connected to the flight controls. Action taken on the flight controls causes the control rod to move, and consequently causes the slide of the associated hydraulic valve to move. As it moves, the slide of the hydraulic valve enables a fluid to flow from the hydraulic valve to the servocontrol, and vice versa. Such a propeller control device is conventional on airplanes. Nevertheless, it should be observed that the distance between the hydraulic valve and the servocontrol is considerable, thereby giving rise to a non-negligible amount of head loss.
Consequently, propeller pitch variation takes place relatively slowly. Thus, the reaction time of the device between a first moment when the pilot gives an order and a second moment when the order has been accomplished is relatively long. This slowness in pitch control is of no problem on an airplane, since the pitch of a propeller is controlled depending on the power delivered by the engine installation of the airplane. Since engine regulation is slow, it is not troublesome for propeller pitch to vary slowly.
The same applies to a hybrid helicopter. When the pilot modifies the power of the engine installation, a computer modifies the pitch of the left and right propeller blades slowly and collectively by the same amount, via an electric motor of a rotary trim actuator, for example. In contrast, in order to obtain yaw control for a hybrid helicopter, the pilot uses the pedals. Unfortunately, in order to counter a gust of wind, or in order to avoid an obstacle, the pilot might impart fast movement to the pedals over a large amplitude.
The assembly comprising the hydraulic valve and the servocontrol then responds slowly, so the pilot's order is not immediately put into effect. Since the pilot does not see the aircraft respond there is a danger of the pilot wondering whether operation is still normal, or whether the flight control is faulty. This leads to a problematic situation in which the pilot might start performing actions that ought not to be performed, e.g. by giving an order contrary to the initial order. The state of the art provides various solutions. For example, document EP 1 348 622 provides for incorporating a damper having a damping coefficient that varies as a function of the rate of variation speed of the flight control. Nevertheless, that factor appears to be insufficient insofar as the amplitude of the movement can also have an impact.